An MMC converter circuit is composed of unit converters that each generate a desired voltage by controlling the modulation factor of a PWM converter that uses an energy storage element with the voltage source characteristics, such as a capacitor or a storage battery, as a voltage source. The voltage of the energy storage element for the unit converter fluctuates according to charging and discharging performed with a period determined by the AC frequency. The unit converters are connected in series to form a two-terminal arm. The first terminals of the two-terminal arms are connected to the terminals for respective phases of an AC power supply and the star-connected second terminals are connected to the terminal of a DC power supply.
With this configuration, the arm connected to each phase generates a voltage with a desired AC frequency to control the alternating current while at the same time superimposing direct current to realize power conversion with the DC power supply.
Control associated with the MMC converter includes current control of adjusting an arm current in accordance with an AC command and a DC command from an external source (such control will hereinafter be referred to as “converter current control” in the present invention); a function of keeping the average voltage of the energy storage element in balance among the unit converters by mutually adjusting the modulation factors of the PWM converters provided in the unit converters within the arms (such a function will hereinafter be referred to as “interstage control” in the present invention); and a function of keeping the total amount of energy stored in the energy storage element in the arm in balance among the arms (such a function will hereinafter be referred to as “interphase balance control” in the present invention). Implementation of the interphase balance control requires a circuit element to reduce current circulating among the arms.
Patent Literature 1 discloses a technology in which a circulating current control reactor is provided between the first terminal of the arm for each phase and the terminal of the AC power supply (this will be referred to as a “DSMMC converter” in the present invention).
Patent Literature 2 discloses a method in which a transformer that includes double star-connected secondary and tertiary windings is provided to cancel out the DC magnetomotive force generated in the transformer core due to the circulating current while using leakage reactance of the secondary and tertiary windings as a current controlling circuit element (this will be hereinafter referred to as a “DIMMC converter” in the present invention).
Patent Literature 3 discloses a method in which a transformer that includes zigzag-connected secondary and tertiary windings is provided to cancel out the DC magnetomotive force generated in the transformer core due to the circulating current while using leakage reactance of the secondary and tertiary windings as a current controlling circuit element (this will hereinafter be referred to as a “ZCMMC converter” in the present invention).
Non Patent Literature 1 discloses a method of connecting the DC terminals of two DSMMC converters back to back as a method of connecting the DC terminals of two MMC converters back to back to be a variable frequency power supply as well as connecting one AC terminal to an AC system and connecting the other AC terminal to an AC rotating electric machine to form a variable speed generator-motor apparatus.
With such a method, direct current is not superimposed in the AC rotating electric machine connected to the MMC converter. This method is thus suitable for varying the speed of the AC rotating electric machine that is connected directly to the AC system and operated at a fixed frequency.
Patent Literature 4 discloses a method of connecting the ZCMMC converter to the side of the AC rotating electric machine. It is argued that, with such a method, an AC electrical system can be implemented without including the circulating current control reactor.
Patent Literature 5 discloses a method of measuring the vectors of variable frequency voltage/current signals of the AC rotating electric machine.